Add configuration file for Read the Docs (#1478)

* Add minimal config file for Read the Docs

* Add instructions to install package; remove docs/requirements.txt

* Fix syntax for extra_requirements

* Use Python 3.8 rather than 3.7

.readthedocs.yaml

@@ -0,0 +1,12 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+python:
+  version: 3.8
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs